In the search for hydrocarbon bearing subsurface earth formations, various systems and methods have been devised for determining parameters which provide indications of such formations and their potential commercial value. One such parameter which is extremely important in assessing the commercial producibility of an oil field is permeability, known in the literature as "k" measured in darcies. The reason for the importance of this parameter is that it is intended to provide a quantitative indication of the expected rate of hydrocarbon flow through the formation and borehole. This in turn is quite important from a commercial standpoint in determining the commercial feasibility of producing a well inasmuch as the formation permeability will be directly related to the potential rate of hydrocarbon production from the well.
Due to the importance of such a parameter, various techniques and apparatus have been devised which sought to arrive at reliable determinations for permeability. Basically, these techniques involved two approaches, namely the analysis of physical core samples of the formation or the interpretation of data derived from logging instruments lowered into the borehole.
With respect to core samples, the desired permeability measurement was made from laboratory testing of the sample. However, in the case of well logging data, permeability was conventionally obtained by an empirical functional relationship between permeability and porosity, the latter of which was sought to be measured by various well logging or "wire-line" techniques.
These aforementioned techniques for seeking to measure the highly valuable permeability parameter were fraught with numerous difficulties. In the case of core sampling, the physical method of obtaining a sample of a subsurface earth formation and retrieving it at the surface for physical analysis immediately suggests many obvious such difficulties. First, there is the expense of such a system in providing for a reliable retrieval mechanism. Secondly, the samples obtained were frequently too small or inaccurate to provide reliable statistical information. For example, it will be appreciated that in the coring process, the formation is damaged and may be contaminated by flushing and invasion of the drilling fluid which may then comprise a portion of the sample retrieved.
Furthermore, by the time the sample is retrieved at the surface and transported to the laboratory for analysis, the sample has undergone many changes which indicate that an inference is not always reliable that parameters measured from the sample are indicative of the actual formation characteristics. For example, hydrocarbons once present in the sample when in place in the formation may have long since left the sample, rendering subsequent tests on the sample unreliable. Still further, only a limited number of discrete samples could be obtained practically, and thus it was often frequently difficult to decide what borehole increment to core.
In the case of well logging data, such techniques likewise suffer from serious deficiencies in attempts to arrive at a reliable determination of permeability. First, it must be appreciated that the effects of hydrocarbons in situ on the various parameters being measured by wireline techniques will be less than those detected by the techniques of the present invention wherein hydrocarbons are detected in their expanded state at the surface. Secondly, wireline techniques exhibit similar difficulties to cure samples in terms of problems presented by a borehole of varying diameter, and drilling fluids therein.
For example, in the various wireline techniques for measuring porosity from which permeability is determined (which may include neutron, acoustic, or induction logging), the technique is to generally measure a parameter from a wireline tool disposed within the borehole wherein the correct parameter sought to be measured is a property out in the formation. Accordingly, this measurement must be made through the borehole wall, drilling fluid, and filtercake. Frequently the wireline data is thus not only measuring the desired formation parameter but also measuring the effects of the borehole and materials therein. In other words, the filtercake, drilling fluids, and the like are adversely affecting the desired formation parameter being measured.
Various techniques have been attempted to alleviate this problem such as borehole compensation wherein a deep measurement into the formation and a shallow one indicative of materials within the borehole are made, the latter being used to compensate for the effects of measuring through these materials in the deep measurement. Nevertheless, difficulties associated with this problem continue to plague the industry. For example, formation density measurements from which porosity and in turn permeability may be determined are typically adversely affected quite obviously by higher drilling mud weights. Moreover, these well logs are also seriously affected by unknown variations in other parameters such as the resistivity of formation water or drilling mud which must be known for accurate formation measurements.
From the foregoing, it is thus not surprising to find that prior determinations of permeability have suffered from numerous deficiencies. First, such permeability values are determined inferentially from questionable data and indirectly from empirical functional relationships between permeability and porosity. Such values for permeability have in many cases been known to be erroneously low, and, accordingly, various "fudge factors" have been known to be introduced in recognition of the unreliability of previous determinations of such permeability.
Even more important, however, is the serious drawback relating to when the questionable permeability measurements are even made available. Because it is necessary to retrieve a core sample for analysis or to cease the drilling operation to permit lowering of the wireline instrumentation package, these important indications of permeability have heretofore not been readily available in real time during the actual drilling operation. Thus, not only would it be highly desirable to have an improved and more accurate technique for making direct determinations of formation permeability, but to be able to do so continuously and in real time during the drilling operation.
These and other deficiencies of the prior art have been overcome by the systems and methods of the present invention, which will be described hereinafter in greater detail with reference to the accompanying drawings.